Sheldon Goldstein had the temerity to write an article with
this title that Physics Today unfortunately printed [1]. In this
article, Goldstein asserts that while "many physicists pay
lip service to the Copenhagen interpretation ... hardly anyone
truly believes" that "quantum mechanics is about
observations or results of measurement." Goldstein is wrong
about that in my case. It seems very simple to me because the
concepts of physics are mutable. I admit that physics is
conceptually very beautiful, and it is hard to maintain a
properly skeptical attitude. However, theoretical physics must
occasionally adjust to new observations that are inconsistent
with old concepts. I hope it happens again in my lifetime.
Goldstein's admonition that "quantum mechanics is
fundamentally about atoms and electrons ..." doesn't specify
what aspect of the existing conceptual framework should be
considered salvageable. For example, Feynman gave a pretty good
account of electrons, but there are still some fundamental
constants in his theory. Those fundamental constants can be
considered one measure of our ignorance about the way nature
works. I believe Feynman himself once said that he had only
"swept a great problem under the rug."

Schreoedinger's Cat Paradox

Goldstein quotes Schroedinger's famous paper [2] on the
"measurement problem" of macroscopic superpositions.
Schroedinger poses his famous cat paradox in a very off-hand way
in this paper. He says it is "quite ridiculous," but he
does not describe in any detail just what is ridiculous about it.
Even if the cat were to be conscious, so what? Goldstein is not a
materialist if he thinks this makes any difference. The quantum
description is correct, in principle, but probably no significant
error will be introduced by assuming that the cat is always
either alive or dead. If somebody could prove otherwise, his or
her contribution would have merit equivalent to that of Bell's
theorem (see below). By the way, Schroedinger made at least one
other error in his paper. He asserts that Dirac's theory
"very strongly transcend(s) the conceptual plan of
Q.M." because of the "stubborn resistance to go
forward to the problem of several electrons."
Goldstein, too wants to press forward with heterodox views even
though they "present formidable difficulties for the
development of a Lorentz-invariant formulation." The reader
must be aware that the combination of quantum mechanics and
general relativity is where the action is now, and Goldstein is
therefore at least 60 years behind the times.

Bell's Theorem

Mermin [3] gave an especially lucid account of Bell's theorem.
Einstein, Podolksy and Rosen had argued that the quantum
description of the decay of a singlet resonance into two
identical particles that have spin is necessarily incomplete. One
horn of the dilemma, as they saw it, is that the spin components
along various directions must have been determined while the two
particles were still close together. In this case, the quantum
description would be incomplete. Bell disproved this. The
alternative for EPR was that, against intuition, the two
particles somehow communicate, even if the two measurements are
made simulataneously. This is because the correlation is perfect
even when the two measurements are separated by a spacelike
interval. Fortunately, this conceptual difficulty does not
suggest any observations or measurements that would violate
relativistic causality. This is why it is necessary to say that
quantum mechanics and physics in general is about observation and
measurement and not about "stuff," which may turn out
to be even stranger than current theories suppose. As they say,
truth is often stranger than fiction.

The Feynman Intepretation

Feynman [4] was very straightforward about the conceptual
problem of quantum mechanics. He said "we cannot make the
mystery go away we will just tell you how it works."
He uses the double slit experiment as an example and discusses it
in great detail because "it contains the only mystery."
He gets to the classical limit by allowing the mass of the
particles to increase, and nothing conceptually different happens
as the wavelength of the particles decreases. However, the
interference fringes become so fine that you cannot see them as a
practical matter. Hence real measurements with macroscopic
particles show the expected smooth curve. I think Feynman would
say that you don't have to "shut up and calculate," if
that is how you characterize the Copenhagen interpretation, but
you do have to calculate. If your detector can resolve the
fringes, you aren't justified in using classical terms with their
implicit assumptions.

A Superposition Experiment You Can Do

Spin states of photons are describable in terms of
polarization, and very good polarizers are available for visible
light. They work by absorbing almost all the photons that are
polarized in one direction and transmitting almost all the
photons polarized at right angles to that direction. Get three
polarizers and cross two of them so that no light comes through.
Then insert the third one between the first two at approximately
45 degrees. Light comes through again, showing that the middle
polarizer erases the initial polarization and creates a new one.
How does a polarizer alter the polarization of a photon without
absorbing it? That is the mystery. I have heard people who should
know better explain it by saying polarizers are highly
birefringent. You can see strain birefringence in a glass object
by putting it between crossed polarizers, but that is not what is
going on here. It is just superposition, pure and simple.